1. Field of the Invention
The present invention relates to an EGR (Exhaust Gas Recirculation) control unit for an internal combustion engine, and more particularly, the present invention relates to an EGR control unit that avoids an increase in a surge (torque fluctuations).
2. Description of Related Art
In Japanese Unexamined Patent Publication No. 2002-048011, surge level index calculating means is provided for calculating a surge level index to detect a surge occurring in the engine on the basis of information related to an angular velocity of an engine crankshaft, and surge-level determining means is provided for determining the surge level from a comparison between the surge level index and a determination threshold. Accordingly, an opening degree of an EGR valve is controlled on the basis of the change in the surge level and the surge level.
In Japanese Unexamined Patent Publication No. 2002-048011, for controlling the opening degree of the EGR valve, particularly a step motor type EGR valve, a basic amount of EGR based on an engine speed and an engine load is multiplied by a limit value based on a target value which is increased/decreased on the basis of a change in a surge level and the surge level, thereby to calculate the amount of the EGR. Then, based on the amount of EGR, a motor target position of the step motor type EGR valve is determined. The step motor type EGR valve is driven according to a comparison result between the motor target position and a current motor position.
Also in Japanese Unexamined Patent Publication No. 2000-199454, an atmospheric pressure and an intake negative pressure are monitored, respectively and a ratio of an exhaust pressure to an intake pressure is calculated and divided by the flow of EGR in order to calculate an effective opening area, thereby estimating a shift in an opening area of the EGR valve (initial stage/degradation).
When the surge increase is detected to correct the amount of the EGR, it is difficult to properly determine a correction amount of feedback unless different correcting methods are used between a case that a tolerance ratio of the EGR due to an environmental change is reduced and a case that the opening area of the EGR valve is shifted (initial stage/degradation). For this reason, in the multiplication correction (gain correction) disclosed in Japanese Unexamined Patent Publication No. 2002-048011, it is possible to deal only with an environmental change. However, it is difficult to deal with the shift of the opening area of the EGR valve. When an operating point is shifted, the surge increase continues until the surge level determination is performed again.
As another method of correcting the opening area of the EGR valve, in Japanese Unexamined Patent Publication No. 2000-199454, the atmospheric pressure and the intake negative pressure are monitored respectively, thereby estimating the shift of the opening area of the EGR valve. However, it is necessary to provide an atmospheric pressure sensor and an intake negative pressure sensor, thus increasing complexity and cost.